Correlations between the presence of X-ray emission and the
morphological composition of groups were suggested from the earliest
ROSAT studies.
Ebeling et al (1994)
were the first to claim such an effect, noting that all but one of the
X-ray detected HCGs in the ROSAT All-Sky Survey data had spiral fraction
less than 50%. Subsequent studies of small samples appeared to support
this trend
(Henry et al 1995,
Pildis et al 1995,
Mulchaey et al
1996a).
However,
Ponman et al (1996)
came to a very different conclusion based on their much larger survey of
the HCGs. They detected several groups with high spiral fractions,
including the extreme example HCG 16, a compact group that contains only spirals.

Figure 7 shows the distribution of early-type
fraction for all the groups with published pointed observations with
ROSAT. For the purposes of this plot, a group is considered "X-ray
detected" only if there is evidence for an extended intragroup medium
component. As is apparent from this figure, a significant number of
spiral-rich groups do contain diffuse X-ray emission, which confirms the
conclusion of
Ponman et al
(1996).
In fact, in contrast to the earlier studies, the distribution of
early-type fractions is surprisingly flat for the X-ray detected
groups. The apparent contradiction with the earlier results can be
explained by the fact that the majority of the groups in the current
sample were selected from optical redshift surveys and were
serendipitously observed by ROSAT
(Helsdon & Ponman
2000,
Mulchaey et al
2000),
whereas the earlier studies were biased toward X-ray luminous groups,
which tend to have higher early-type fractions
(Mulchaey &
Zabludoff 1998).

Figure 7. Distribution of early-type
fraction for all groups (open histogram) and groups with diffuse
X-ray emission (shaded histogram). The top panel shows the
result for all published PSPC pointed-mode observations, whereas the
lower panel contains only groups selected from optical redshift
surveys.

A closer examination of Figure 7 reveals that
while many spiral-rich systems are X-ray sources, spiral-only groups
tend not to contain a diffuse X-ray component. The one exception in
Figure 7 is HCG 16. However, the true nature of
the X-ray emission in HCG 16 is unclear. The ROSAT image of the group
indicates that the emission is very clumpy and concentrates around the
brightest group members (see
Figure 2, top). Some authors
have attributed all of the X-ray emission to individual galaxies
(Saracco & Ciliegi
1995;
see also an earlier Einstein observation by
Bahcall et al 1984),
whereas others have claimed the existence of intragroup gas
(Ponman et al
1996).
Dos Santos & Mamon
(1999)
have reanalyzed the ROSAT PSPC data on HCG 16, paying special attention
to the removal of emission associated with galaxies. Although
Dos Santos & Mamon
(1999)
derived a lower luminosity for the diffuse gas than
Ponman et al
(1996),
they still found evidence for some diffuse gas. However, the presence of
diffuse emission does not necessarily mean that HCG 16 contains a
diffuse intragroup medium. One possibility is that the emission is
related to the unusually high number of active galaxies in the group
(HCG 16 contains one Seyfert galaxy, two LINERs, and
three starburst galaxies;
Ribeiro et al
1996).
The X-ray to infrared luminosity ratio of this system is much higher
than one would expect if the X-ray emission is related to the galaxies'
activity, however (Ponman, private communication). Alternatively, the
X-ray emission may be associated with shocked gas, as appears to be the
case in Stephan's Quintet
(Pietsch et al 1997).

With the possible exception of HCG 16, all X-ray detected groups studied
to date contain at least one early-type galaxy. There are several
possible explanations for why spiral-only groups do not contain diffuse
X-ray emission. One possibility is that all spiral-only groups are
chance superpositions and not real, physical systems. This possibility
seems unlikely, given the existence of our own spiral-only Local Group
(see Section 5.10 for a discussion of
the intragroup medium in the Local
Group). Another possibility is that the intragroup gas in spiral-only
groups is too cool to produce appreciable amounts of X-ray emission
(Mulchaey et al
1996b).
Based on velocity dispersions, the virial temperatures of spiral-only
groups do tend to be lower than those of their early-type dominated
counterparts
(Mulchaey et al
1996b).
While a cool (i.e. several million degrees K) intragroup medium would be
difficult to detect in X-ray emission, such gas might produce prominent
absorption features in the far-ultraviolet or X-ray spectra of
background quasars
(Mulchaey et al
1996b,
Perna & Loeb
1998,
Hellsten et al
1998).
In fact, several such groups may have already been detected as OVI
1031.93, 1037.62
Å absorption systems
(Bergeron et al
1994,
Savage et al 1998).
A third possibility is that the gas densities in spiral-only groups are
too low to be detected in X-rays. Low gas densities in spiral-only
groups are in fact consistent with recent prediction of preheating
models for groups
(Ponman et al 1999;
see Section 5.9).